As set forth by D. Geldart, Types of Gas Fluidization, Power Technology, 7(1973) 285-292, the behavior of fluidized beds falls into easily recognizable groups depending on the density difference between the gas and the solids and mean particle size. Group A powders exhibit dense phase expansion after minimum fluidization and prior to the commencement of bubbling. Group B bubble at the minimum fluidization velocity. Group C are difficult to fluidize at all and those in group D can form stable spouted beds. In another publication, Geldart and Wong, Fluidization of Powders Showing Degrees of Cohesiveness--I. Bed Expansion, Chemical Engineering Science, Vol. 39, No. 10, pp. 1481-1488 (1984), describes the Hausner ratio, which is the ratio of the tapped bulk density to loosely packed bulk density, as giving a good indication of the cohesiveness of the powder.
The present invention is concerned with heat transfer in fluidized beds of Geldart group C powders which are cohesive and difficult to fluidize. In general, the rate at which heat is transferred between the wall of a fluidized bed and the fluidized powder or particles is given by the equation: EQU Q=hA(.DELTA.T)
where
Q is the rate of heat transfer, W PA1 h is the wall to bed heat transfer coefficient, W/m.sup.2.K PA1 A is the area of the bed in contact with the heated wall, m.sup.2 PA1 .DELTA.T is the temperature difference between the wall and the bed, K.
Heat often needs to be transferred from the wall to the particles in a fluid bed reactor to maintain the bed temperature at the optimum level for thin film deposition on cohesive powders like phosphors. For example, U.S. Pat. No. 4,825,124 to Sigai, in column 7, lines 32 to 37, describes the use of a resistance heated zone furnace for heating a fluidized bed of cohesive particles, and, in column 3, lines 53 to 55, describes nitrogen, argon, helium, neon or mixtures thereof as examples of inert gases "suitable for use". There is no discussion, however, of the heat transfer characteristics of the bed. As opposed to bubbling fluidized beds which are noted for high values of h, fluid cohesive powders are handicapped by low values of h since they do not have bubbles to induce large scale particle motion. This results in a problem: a large wall area A is required to transfer the needed Q into the bed for a practical value of .DELTA.T. A large wall area may be realized by increasing the diameter and/or the height of the column, both of which pose difficulties in the case of cohesive powder fluidization. In particular, a larger bed diameter makes heat transfer to volume elements near the center of the column more of a problem. In addition, a longer bed height increases the length of the channels of gas and makes it more challenging to break up these channels. Any improvement in cohesive fluid bed operation wherein the value of h can be increased is, therefore, a major advancement in the fluid bed CVD of thin films on cohesive powders.
While the above situation is described in conjunction with the heating of fluidized beds, there are instances where heat may have to be extracted from the cohesive powder fluidized bed. For example, the region of a bed close to the porous gas distributor may have to be cooled to prevent decomposition of a reactant and plugging of pores. As another example, it is often desirable to cool fluidized beds where exothermic reactions are occurring to prevent degradation of the bed materials or the initiation of undesired side reactions. In such cases, any improvement in cohesive powder fluid bed operation wherein the value of h can be increased is a major attraction since a smaller heat transfer area with its associated advantages of more economical heat transfer may be employed.
There is a considerable amount of literature on heat transfer characteristics of bubbling fluidized beds. An excellent summary by Xavier and Davidson (Fluidization, Second Edition, Academic Press, London, 1985) is attached with this specification. The bubbling fluidized bed behavior of Geldart type A and B powders is totally different from that of cohesive Geldart type C powder fluidized beds. The fluidization of cohesive, Geldart type C powders like phosphors, electrostatic toners, ultra fine ceramic powders, fine pharmaceutical powders, etc. is characterized by low powder mobility due to the bypassing of the fluidizing gas via the formation of channels. The channels are a network of vertical and inclined cracks. See Dutta et al., AIChE Symp. Ser. 276(86), 26, 1990; AIChE Symp. Ser. 281(87), 38, (1991).
The formation of channels of gas in fluidized beds of cohesive Geldart type C powder, instead of bubbles, is attributed to the dominance of inter particle forces over fluid dynamic drag forces. The presence of channels means that the entire bed weight is not supported by the pressure drop of the gas. The pressure drop of the fluidizing gas in cohesive powder fluidization is, therefore, smaller than the bed weight. The ratio of the two, called the normalized bed pressure drop, is thus smaller than unity for such systems while it is practically unity for bubbling beds. The more severe the bypassing of gas, the larger is the deviation in the normalized bed pressure drop from unity in cohesive powder fluid beds. Bubbling fluidized beds differ, therefore, from cohesive powder beds in that the latter have no bubbles, have lower than unity normalized bed pressure drop and have low powder mobility. The lack of bubbles and poor powder motion in such systems leads to very low values of the heat transfer coefficient h as compared to bubbling beds.
Due to such intrinsic differences between bubbling beds and cohesive powder fluidized beds, the heat transfer correlations and concepts developed in the literature for bubbling beds are not necessarily applicable to cohesive powder systems. All heat transfer information in the literature for bubbling systems are based on the presence of bubbles which are absent in cohesive powder fluidization.
Any improvement which significantly enhances the heat transfer characteristics in a fluidized bed of Geldart type C cohesive powder represents an advance in the art.